1. Field of the Invention
The present invention relates to a Ferraris sensor for measuring a parameter of a moving object, such as an acceleration of the moving object.
2. Description of the Related Art
Velocity or acceleration sensors based on the principle of eddy current measurements are also called Ferraris sensors. Exterior magnetic fields generate eddy currents in a moving, electrically conducting, non-ferromagnetic body, whose strength is a function of the relative speed between the exterior magnetic field and the moving body. Interior magnetic fields, which are overlaid on the exterior magnetic fields, are linked to these eddy currents. If the strength of these interior magnetic fields is detected, for example by Hall sensors, a signal is obtained which is proportional to the velocity of the moving body.
Accelerations, i.e. changes in the velocity of the moving body, change the interior magnetic field linked to the eddy currents, so that voltages which are proportional to the acceleration, are induced in detector coils.
Detectors operating in accordance with the Ferraris principle thus permit the direct measurement of a velocity or an acceleration.
Such an eddy current detector is known from DE 37 30 481 A1, which can be used as a tachometer or acceleration meter. It contains a non-magnetic electrically conductive body, whose velocity or change in velocity is to be measured. A constant magnetic field is generated substantially perpendicular with respect to the movement direction and leads to eddy currents in the moved body, which in turn cause an eddy current magnetic field. When employed as a tachometer, the flux density of the eddy current field is measured by a Hall effect sensor, whose output signal is proportional to the velocity. When employed as an acceleration meter, the change over time of the flux density of the eddy current effect is measured by a coil, whose output voltage is proportional to the acceleration.
The large amount of space as a result of the use of an encompassing yoke structure is disadvantageous. Moreover, the yoke in addition also prevents the employment of the sensor for measurement on a body which can be freely displaced on a level, since one direction at least is limited because of the yoke.
An eddy current detector which does not use an encompassing yoke, is proposed by JP 61223564. Three magnetic poles are created by two coils, through which a dc current flows in opposite directions, starting from which an exterior magnetic field penetrates an eddy current disk. In this case the magnetic poles are arranged one behind the other in the direction of the movement to be detected. However, the arrangement of the coils does not permit an optimal detection of the generated eddy currents and therefore no satisfactory signals can be obtained.